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March 26, 2017

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A mixture of chromium and zinc weighing .362 g was reacted with an excess of hydrochloric acid.
After all the metals in the mixture reacted 225 mL of dry hydrogen gas was collected at 27 C and 750 torr.
Determine the mass percent Zn in the metal sample. [Zinc reacts with hydrochloric acid to produce Zinc chloride and hydrogen gas;
chromium reacts with hydrochloric acid to produce chromium (III) chloride and hydrogen gas.]

Let Y = grams Zn
Z = grams Cr
==============
Y+Z=0.362 g
Y(MM Zn/MM H2) + Z(2MM Cr/3MM H2) = grams hydrogen.
=====================
Note: MM stands for molar mass. 2MM and 3MM stands for 2xmolar mass and 3xmolar mass respectively.
To find grams hydrogen:
225 mL at 27C (300 Kelvin) and 750 torr will correct to STP as zz mL at 273 K and 760 torr. Then change to liters and L/22.4 = mols and mols x molar mass hydrogen (which is 2) will convert to grams.

Then solve the above set of simultaneous equations for Y to obtain grams Zn.
Then grams (Zn/0.362)x100 = % Zn

Check my thinking. Check my work.

The second equation in the set comes from the balanced equations.
Zn + 2HCl ==> H2 + ZnCl2
2Cr + 6HCl ==> 3H2 + 2CrCl3

I hope this helps.

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